bash - sed: replacing end of line -

i need quote end of line , using this solution replace end of line working when file has multiple lines.

problem arises when try use method files have no \n or have single line ending \n. in case sed not replace anything, second s/ command not work. looks last end of line never replaced.

my command is:

sed ':a;n;$!ba;s/\n/\\n/g;s/a/_/g' <file> 

(replace \n \\n , replace a _ example).

i use hexdump -c display files here:

there tests made:

$ # test1. single line without \n $ echo -n 'abc' | sed ':a;n;$!ba;s/\n/\\n/g;s/a/_/g' | hexdump -c 00000000  61 62 63                                          |abc| # expected "_bc"  $ # test2. single line \n $ echo 'abc' | sed ':a;n;$!ba;s/\n/\\n/g;s/a/_/g' | hexdump -c 00000000  61 62 63 0a                                       |abc.| # expected "_bc\n"  $ # test3. 2 lines $ echo -e 'abc\n' | sed ':a;n;$!ba;s/\n/\\n/g;s/a/_/g' | hexdump -c 00000000  5f 62 63 5c 6e 0a                                 |_bc\n.| # expected "_bc\n\n" 

questions: why second s/ not replace in test1 , test2? there method fix replacing of s/\n/\\n/ , s/a/_/g in tests?

p.s.: don't want workaround adding newline @ end of stream before sed processing , removing after.

edit: looks n command not read single line, if followed '\n'. idea how fix it?

edit2: seems expected behavior of n command. there method on how replace last line ending?

this might work (gnu sed):

sed ':a;$!{n;ba};s/\n/\\n/g;s/a/_/g' file 

or:

sed ':a;$!n;$!ba;s/\n/\\n/g;y/a/_/' file 

logically makes sense: 1 cannot next line if on last line.